This specification relates to an optical device and in particular to an optical depolarizer.
Along with the development of network communications and increasingly fast data transfer rate over optical fiber networks, the requirement for capacities of optical fiber networks becomes higher and higher. Most signals transmitted over existing optical fiber networks are polarized. Wear and tear of devices related to the polarization, gains related to the polarization, and other interference sensitive to polarized light all affect the quality of signals transmitted over optical fiber networks. Therefore, some conventional optical fiber networks use optical depolarizers to convert polarized light signals to non-polarized light signals.
Common optical depolarizers include Lyot depolarizers, which have the structure shown in FIG. 1. A Lyot depolarizer 100 includes two segments of birefringent crystals or birefringent optical fibers. In the case of birefringent crystals, the Lyot depolarizer 100 shown in FIG. 1 has a first birefringent crystal 11 and a second birefringent crystal 12, the tail end of the first birefringent crystal 11 connects with the head end of the second birefringent crystal 12. Moreover, the length of the first birefringent crystal 11 is L1, the length of the second birefringent crystal 12 is L2, and L2 is greater than L1, e.g., L2 can be twice as long as L1.
FIG. 2 illustrates a principal birefringent axis at an emitting end of the first birefringent crystal 11 of FIG. 1 and an entering end of the second birefringent crystal 12 in the optical depolarizer of FIG. 1. As shown in FIG. 2, on the connection surface between the first birefringent crystal 11 and the second birefringent crystal 12, the principal axis direction Y of the first birefringent crystal 11 forms a 45° angle with the principal axis direction Y′ of the second birefringent crystal 12.
FIG. 3 illustrates a delay of light components at sequential spatial positions when a light beam passes through the optical depolarizer according to the prior art. As shown in FIG. 3, after entering an incident end of the first birefringent crystal 11, incident light u(0) is split into two light components having mutually perpendicular polarization states in the optic axial direction: ordinary light (o light) and extraordinary light (e light), which are expressed as ux(0) and uy(0), respectively, in FIG. 3, as identified by reference 300.
The two light components, ux(0) and uy(0), have different indexes of refraction relative to the first birefringent crystal 11, which are referred to as o light index of refraction and e light index of refraction. As a result, the two light components, ux(0) and uy(0), have different propagation speeds inside the first birefringent crystal 11. After passing the length L1 of the first birefringent crystal 11, the two light components, ux(0) and uy(0), become light components, ux(L1) and uy(L1) (reference 302), and a time difference is generated between the two light components, ux(L1) and uy(L1). The delay τ1 is expressed by Equation 1 as:τ1=(Δn/c)×L1  (Equation 1)In Equation 1, Δn is the difference between the o light index of refraction and the e light index of refraction, and c is the light speed.
Similarly, after entering the second birefringent crystal 12, the incident light ux(L1) is further split into two light components, uxx′(L1) and uxy′(L1). Light components Uxx′(L1) and Uxy′(L1) have mutually perpendicular polarization states along the directions that are perpendicular and parallel to the optic axis. After passing the length L2 of the second birefringent crystal 12, the light components, uxx′(L1) and uxy′(L1), become light components, uxx′(L2) and uxy′(L2) (reference 304).
After entering the second birefringent crystal 12, the incident light uy(L1) is further split into two light components, uyx′(L1) and uyy′(L1), having mutually perpendicular polarization states along the directions that are perpendicular and parallel to the optic axis. After passing the length L2 of the second birefringent crystal 12, the light components, uyx′(L1) and uyy′(L1), become light components, uyx′(L2) and uyy′(L2) (reference 306).
Due to the presence of a time difference between the propagation of light components in different polarization states in a birefringent crystal, a delay is generated. For example, the delay between the light components, uxx′(L2) and uxy′(L2), is τ2, where τ2 is calculated as:τ2=(Δn/c)×L2.
Typically, to meet demand for converting polarized light into non-polarized light, it is necessary for all light components that reach the emitting end of the second birefringent crystal 12 to be incoherent to each other, i.e. it is required that:
τ1≥τc and τ2−τ1≥τc, wherein τc is the coherent time of the propagating light. As a result, the length L1 of the first birefringent crystal 11 and the length L2 of the second birefringent crystal 12 need to satisfy the following constraint requirement:L1≥(τc×c)/Δn  (Equation 2);L2≥2×L1  (Equation 3)
However, conventional communication systems often use light sources with relatively narrow spectral distribution, such as Raman amplifiers, etc. Due to the small difference Δn in the index of refraction between the o light and the e light in a birefringent crystal, a typical depolarizer requires two birefringent crystals with the total length above one hundred millimeters to realize depolarization. Because of the high cost of birefringent crystals and difficulties in production and assembly of birefringent crystals with such a length, such depolarizers are typically only suitable for depolarizing light sources with a wide spectrum, but typically cannot be used for depolarizing common quasi-monochromatic light with relatively long coherent length. Consequently, the application scope of this conventional depolarizer is greatly limited.
Another conventional depolarizer uses a Wollaston prism as a wave-combining component and disposes a depolarization wave-plate at an emitting end of the Wollaston prism. The thickness of the depolarization wave-plate needs to satisfy the constraint requirement that an optical path difference between fast axis light and slow axis light is greater than the coherent length of the light source.
To make a depolarization wave-plate thick enough to satisfy the above depolarization requirement, the thickness needs to be very long for regular birefringent crystals, such as yttrium vanadate crystal. As a result, this depolarizer has a large volume and moreover, a high production cost.
FIG. 4 is a plot 400 of a spectrum of wavelength and power gain of a Raman pump light source. A light source produced by a Raman pump laser typically contains multiple spectral peaks having different wavelengths. All of these beam components are linear polarized light with a relatively high degree of polarization. Moreover, the power of these beam components varies along with the changes in wavelength, as shown in FIG. 4. A power change curve of a Raman pump light source has periodic peaks and valleys along the wavelength axis. The wavelength difference of the two neighboring peaks is identified by δλ. The wavelength difference δλ is essentially equal for the wavelengths that every two neighboring peaks correspond to, and the wavelength difference δλ is very small and typically at the nanometer level or smaller.
To depolarize Raman pump light sources, a Raman pump light is split into two beams each having equal energy and mutually perpendicular polarization states. One of the beams is made to travel an extra optical path such that an optical path difference is generated between the two beams. The optical path difference can ensure that beam component phases of wavelengths that neighboring peaks correspond to are different by half a cycle or an odd number multiple of half a cycle.
FIG. 5 illustrates a plot 500 of a decay of the polarization degree of a Raman pump light in Stokes space. As shown in FIG. 5, when expressed in the Stokes space, phases of all neighboring peaks are different by π or an odd number multiple of π when emitting, the vector sum thereof will cancel each other, i.e. the composite optical energy over the entire band can be depolarized.
The principle of the above method is to make the phase of a beam at a wavelength in a Raman pump light source different from the phase of the beam at the wavelength of the neighboring peak by an odd number multiple of half a cycle, which can be expressed with the following equation:Γ(λ)−Γ(λ+δλ)=(2m+1)π  (Equation 4).
In Equation 4, Γ(λ) is the phase of the beam with a wavelength of λ, Γ(λ+δλ) is the phase of the beam with a wavelength of λ+δλ, m is an integer greater than or equal to zero, and π is the phase angle of half a cycle.
In addition, existing light sources require depolarization processing after combining a plurality of beams. When beams of some light sources are emitted, a subsequent system may partially back reflect the beam into the light source to affect the light source. There is a need to isolate the back reflected beam so as to prevent the subsequent system from reflecting the beam into the light source.